1. Field of the Invention
The present invention relates to photographic printers. In particular, the present invention is an improved method and apparatus for controlling exposure times based upon large area transmission density sensor readings.
2. Description of the Prior Art
Photographic printers produce photographic prints from photographic film originals (generally negatives) by passing high intensity light through the film to expose the photographic emulsion layers of a photosensitive medium. The emulsion layers are subsequently processed to produce a print of the scene contained in the original.
In order to increase efficiency and minimize time required to fill customer orders, high speed printers have been developed in which many exposures are made in edge-to-edge relationship on a single roll of print paper. After the exposures are made, the roll is removed from the printer, is photoprocessed to produce prints, and is cut into individual prints. The prints are then sorted by customer order and ultimately packaged and sent to the customer.
A critical portion of a photographic printer is the exposure time control, which controls the duration of the exposure of the photosensitive medium. The exposure time control may utilize inputs from several different sources in order to determine the proper duration of the exposure. For example, most automatic printers use large area transmission density (LATD) sensors to sample the light transmitted by the negative either prior to or during the exposure. In addition, many automatic printers include an automatic density correction (ADC) or color scanning station which scans the negative prior to printing and corrects the exposure time in the event of an abnormality in illumination of the negative known as "subject failure". The operator may also enter density or color correction signals from the operator control panel. Based upon some or all of these input signals, the exposure time control determines the proper exposure time for each of the color channels based upon predetermined exposure time relationships.
In general, most printers which use LATD sensors are a single sensor reading for each color to determine the exposure times during a particular exposure cycle. In many analog and some digital controlled automatic printers, the LATD sensor readings are taken during the exposure cycle itself. In other digital controlled photographic printers, the LATD sensor readings are taken prior to the commencement of the exposure cycle, and the exposure times for all three color channels are calculated based upon the sensor values, automatic density correction signals (if any), operator entered correction signals, and other stored data prior to initiation of the exposure cycle. In both types of systems, the accuracy of the exposure times is a function of the accuracy of the LATD sensor readings.